When island means 'a mass of land surrounded by water', or is part of a name, the usual preposition is on (NOT in ): 'The prisoners were left on a small island, with neither food nor drinking water.' 'Mr Mandela was placed in a maximum security prison on Robben Island.'

When an island is considered in terms of its people, cities, schools, roads, traditions, economy etc, both in and on are used: 'There has been no serious outbreak of cholera in the island for over twenty years.' 'Elsewhere in the island most of these stone cottages have disappeared.'